Class mapping can have "discriminator-value" as well as subclass, but there is no way to specify it in automapping class convention inherited from IClassConvention. Reason for this is lack of DiscriminatorValue setting method in ClassInstance class and it's corresponding IClassInstance interface.
Problem can be solved by adding following method in ClassInstance class, and it's signature in IClassInstance interface
public new void DiscriminatorValue(object value)
mapping.Set(x => x.DiscriminatorValue, Layer.Conventions, value);
P.S. Method taken from similar SubclassInstance class